The present invention relates to an engine assembly and to a waste heat recovery system.
The engine assembly may include an internal combustion engine which is provided with a turbocharger. Turbochargers are well known devices for supplying air to an inlet of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
The turbine of a conventional turbocharger comprises: a turbine chamber within which the turbine wheel is mounted; an annular inlet defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the annular inlet; and an outlet passageway extending from the turbine chamber. The passageways and chamber communicate such that pressurised exhaust gas admitted to the inlet volute flows through the inlet to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet so as to deflect gas flowing through the inlet. That is, gas flowing through the annular inlet flows through inlet passages (defined between adjacent vanes) which induce swirl in the gas flow, turning the flow direction towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that characteristics of the inlet (such as the inlet's size) can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the inlet using a variable geometry mechanism. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.
A waste heat recovery system may be arranged to recover heat from engine exhaust gases after they have passed through the turbine of a turbocharger (and may also be arranged to recover heat from exhaust gases which have not passed through a turbocharger). The waste heat recovery system may be arranged to convert the recovered heat into usable power. This may be done for example by converting the recovered heat into electricity, or may be done by converting the recovered heat into power which augments an output of an engine.
A conventional waste heat recovery system uses a refrigerant fluid which is pumped around a closed loop. A heat exchanger is used to transfer heat from exhaust gases to the refrigerant, which is initially in liquid form. This heat causes the refrigerant liquid to vaporise. The resulting refrigerant gas passes to a turbine and drives a turbine wheel of the turbine to rotate, thereby allowing usable power to be derived. The refrigerant gas passes from the turbine to a condenser which is configured to cool and condense the refrigerant so that it returns to liquid form. The refrigerant liquid is then passed to the heat exchanger, where the heat recovery cycle begins again.
The turbine wheel of the waste heat recovery system is mounted on a shaft which is held in a housing. A bearing is provided between the shaft and the housing, the bearings being configured to allow the shaft (and the turbine wheel) to rotate freely. Oil is delivered to the bearings through the housing, the oil acting to ensure that the bearings can move freely. A problem which may arise with this arrangement is that oil may pass from the bearings into the refrigerant fluid. This is undesirable because heat exchangers of the refrigerant fluid loop may be at high temperatures, and the oil may cause damage to the heat exchangers when it is heated to those high temperatures. In order to avoid such damage occurring it is conventional to include a separation apparatus which is configured to separate oil from the refrigerant fluid. However, such separation apparatus may be complex and expensive.